In internal combustion engines, a process known as exhaust gas recirculation (EGR) is used to reduce the amount of nitric oxide (NO.sub.x) emissions. In general, EGR involves routing a portion of the exhaust gas back into the intake air flow. In an engine where exhaust back pressure is greater than intake air pressure (e.g., most normally aspirated engines) an EGR flow can be realized simply by connecting a conduit between the exhaust and intake ducts. The flow from the exhaust manifold is drawn to the lower pressure of the intake because of the negative pressure differential. However, in a turbocharged internal combustion engine having a charged intake, an unfavorable pressure differential must be overcome.
More particularly, in turbocharged diesel engines, the recirculated exhaust gas flow is typically introduced into the intake air downstream of the turbocompressor and intercooler components in order to avoid degradation of these components. Unfortunately, the intake air is pressurized at this location, presenting an unfavorable intake to exhaust pressure ratio for transporting EGR gases. Diesel engines with efficient, well-matched turbochargers have an insufficient exhaust-to-intake pressure differential during some operating states to induce an amount of EGR flow sufficient to reduce nitric oxide emissions in compliance with environmental emissions regulations. For example, when an engine is running at a low speed under a high load, the exhaust temperature can be hot enough that the intake manifold pressure is higher than the exhaust manifold pressure. If a connection is opened between the intake manifold and the exhaust manifold through the EGR system, air flows from the intake manifold into the exhaust manifold, rather than the desired flow from the exhaust manifold to the intake manifold.
Various EGR systems are known which attempt to overcome this unfavorable pressure differential via a supplemental pump or compressor operable to force flow through the EGR duct from the exhaust manifold to the intake manifold. Such EGR compressors can be mechanically driven, electric, or exhaust-driven by a second turbocompressor. It is also known to provide a supplemental compressor in the EGR system to drive flow. However, they require a relatively large amount of energy which tends to substantially sacrifice fuel economy. These systems also increase the complexity and cost of assembling an engine.
Systems are also known which attempt to achieve exhaust-to-intake EGR flow by increasing the pressure in the exhaust manifold (backpressure). This can be achieved with careful engine design choices, such as the selection of a turbocharger having a particular fixed-geometry turbine size to yield an appropriate pressure differential. To increase EGR flow in a particular engine, a fixed-geometry turbocharger could be selected to have a relatively small size, having a correspondingly small swallowing capacity and a high flow restriction. The high flow restriction would provide increased backpressure upstream of the turbine sufficient to drive the EGR flow back to the intake manifold. Unfortunately, a turbocharger turbine and compressor selected to provide such backpressure are poorly matched, and such a system results in less-than-optimal performance, sacrificing fuel economy and power, especially at higher engine speeds.
In conjunction with the relatively small sized turbine discussed in the aforementioned paragraph, it is further known to provide a wastegate for bypassing excess amounts of exhaust backpressure in an attempt to thereby increase engine power at relatively high speeds. The wastegate bypass typically includes a modulating valve which modulates exhaust flow therethrough at a frequency synchronous with exhaust pulses. Although engine efficiency and power are improved overall, this solution still results in a smaller turbine being selected for the engine than is desired for optimal performance and fuel economy. Furthermore, the modulating valve of the wastegate bypass has been known to experience reliability problems due to the harsh engine environment in which it operates. In particular, the modulating valve of the wastegate bypass typically operates in close proximity to the engine's combustion chambers where exhaust temperatures, gas pressures and engine vibrations are greater. These factors decrease control over the wastegate valve which in turn decreases control over EGR flow and the engine in general.
In an attempt to overcome the problems of wastegated turbines having a small, fixed swallowing capacity, it is known to provide a variable geometry turbine with movable vanes that can be adjusted to vary the swallowing capacity and flow resistance of the turbine. However, this solution is only viable in situations where cost is not prohibitive, because variable geometry turbines are typically more expensive than wastegated turbines. Moreover, in these systems the boost and rotor speed provided by the turbine become dependent upon swallowing capacity of the turbine which results in less than desirable engine performance at various engine operating conditions.
Given that emissions regulations are increasingly becoming more stringent, an improved EGR system is needed.